The invention relates to a zinc oxide varistor.
It is known that the properties of zinc oxide varistors deteriorate markedly after a relatively short time, if they are operated in oxygen-free surroundings. In particular, the leakage current and hence the energy absorption increase, and this leads to heating-up and consequently to a further increase in the leakage current and, as the final outcome, to thermal destruction of the varistor. It has been found that the degradation of the varistor material occurs principally in a thin lateral edge layer, and it is therefore suspected that it is to be ascribed to the formation of less than stoichiometric zinc oxide as a result of oxygen diffusing out of the edge layer into the surroundings of the varistor.
DE No. 3,123,552 A1 discloses a varistor which, in order to prevent or slow down the out-diffusion of oxygen, is provided, on the side face going round between its contact surfaces, with a coating of an organic polymer, which should be gas-permeable.
In fact, at the usual operating temperature of zinc oxide varistors, namely about 120.degree.-130.degree. C., the oxygen permeability of organic polymers is in general fairly high and it appears doubtful whether a pure polymer coating can form an oxygen barrier which substantially slows the degradation of the zinc oxide varistor.
From U.S. Pat. No. 3,959,543, it is also known to provide zinc oxide varistors with a glass coating on the side. As is clear, particularly from the above mentioned patent, glass having a coefficient of thermal expansion which is sufficiently close to that of the varistor material has a relatively high melting point. In order to apply the glass coating, the varistor must, after it has been finished, be heated once more to about 650.degree. C., and this can have unfavorable effects on its electrical properties. Further disadvantages are the brittleness of the glass coating, making handling of the varistor more difficult, and the fact that the coating can, under certain circumstances, be chemically corroded by hydrofluoric acid.
According to Japanese Patent Specification No. 957,072, the degradation of zinc oxide varistors in SF.sub.6 can be substantially slowed down by an addition of 10-30% of oxygen. Apart from the fact that it is not applicable if the varistors are surrounded by a liquid or solid medium, this method has the disadvantage that it requires gas-tight sealing of the space containing the varistors from other parts of the equipment and makes maintenance more complicated. In addition, the dielectric strength is reduced.